Biology · Excretory Products and Their Elimination · NEET
The ascending limb of the loop of Henle is impermeable to water but pumps out NaCl into the medullary interstitium. The collecting duct also leaks a small amount of urea into the inner medulla. Together, salt plus urea make the interstitium reach about 1200 mOsmol/L in the inner medulla, four times the plasma value (300 mOsmol/L). This high salt zone is the key that allows water to be pulled out later.
In the loop of Henle, filtrate flows down the descending limb and up the ascending limb in opposite directions. This opposite flow, plus the U-shaped vasa recta blood flowing the opposite way, traps the salt inside the medulla instead of washing it away. So the counter current does not concentrate the urine directly; it builds and maintains the 300 to 1200 mOsmol/L gradient that makes concentration possible.
The vasa recta is a U-shaped blood vessel running parallel to the loop of Henle. Blood enters and leaves the medulla close together, flowing in opposite directions (counter current). As blood goes down it picks up salt, and as it comes up it gives most of that salt back. This prevents the blood from carrying the salt away, so the medullary gradient stays intact.
In the collecting duct. The collecting duct passes from the cortex deep into the salty inner medulla. When ADH is present, the duct wall becomes permeable to water, so water moves out of the filtrate by osmosis into the hyperosmotic interstitium. The remaining fluid becomes concentrated urine (up to about four times the plasma osmolarity).
Both are needed. The counter current mechanism builds the salty medulla (the pulling force). ADH (vasopressin) opens water channels in the collecting duct so water can respond to that force. High ADH plus a strong gradient gives concentrated urine; low ADH keeps the duct closed to water and gives dilute urine even if the gradient exists.
Which of the following factors is responsible for the formation of concentrated urine?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The hyperosmotic (very salty) inner medulla, about 1200 mOsmol/L, created and maintained by the counter current mechanism. Water leaves the collecting duct into this salty zone, concentrating the urine.
About 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla, roughly a four-fold gradient. NCERT gives these exact numbers, so remember them for NEET.
The loop of Henle and the vasa recta, both running as counter current systems. The loop pumps out and traps salt; the vasa recta keeps the salt from being washed away; urea from the collecting duct adds to it.
No. It builds and maintains the salt gradient in the medulla. The actual water removal happens in the collecting duct, driven by that gradient and controlled by ADH.
The collecting duct stays impermeable to water, so water is not reabsorbed and dilute urine is formed, even though the medullary gradient still exists.
